I am also using 18.8 on old WinXP laptop. The key thing to keep to free version is to click "No Thankyou" to all trials offered. I think you have said you did that, but then ended up with no reactivation.
I wonder if it would be worth a repair or complete uninstall (using the right uninstaller for that version) and then reinstallation of 18.8.
I believe that is what I had to do on one occassion, but don't think my reason was particularly to renew free licence.